1. Field of the Invention
The present invention relates to a semiconductor device using a thin film transistor (TFT) mounted on an insulating substrate such as a glass plate, and more particularly to a semiconductor device which can be utilized in an active matrix type liquid crystal displaying unit, or the similar matrix circuit.
2. Description of the Related Art
An active matrix type liquid crystal display unit using a TFT to drive a pixel, an image sensor, a three dimensional integrated circuit, and the like are known as a semiconductor device having a TFT on an insulating substrate such as a grass plate.
A thin film silicon semiconductor is generally used as the TFT mounted on such a device. In particular, for a high speed operation it is strongly required to establish a method for manufacturing a TFT comprising a crystalline silicon semiconductor. A method of conducting crystallization by forming an amorphous semiconductor film and applying a heat energy thereto (heat annealing) is known as a method for obtaining such a crystalline thin film silicon semiconductor.
There are some problems in manufacturing a semiconductor circuit using the crystalline silicon film thus formed. For example, a circuit that not only a matrix circuit but also the peripheral circuit for driving the same are constituted of the TFT (monolithic type active matrix circuit) is taken into account as an active matrix type circuit used in a liquid crystal display unit (i.e., a circuit that a controlling transistor is arranged in each pixel).
In this complicated circuit, characteristics required in the TFT vary depending on the position of the circuit. For example, the TFT used for controlling the pixel of the active matrix circuit is required to have sufficiently small leak current in order to maintain an electric charge stored in a capacitor constituted of a pixel electrode and an opposite electrode. However, a current driving ability may not be so high.
On the other hand, a large current switching at a short time is necessary in the TFT used in a driver circuit which supplies signals to a matrix circuit, and the TFT having a high current driving ability is required. However, a leak current may not be so low.
A TFT having a high current driving ability and a low leak current is most desirable. However, the TFT presently manufactured is far from such an ideal TFT, and if the current driving ability is high, the leak current is also high, and if the leak current is low, the current driving ability is low.
Therefore, the monolithic type active matrix circuit constituted using the conventional TFT attempts to improve the current driving ability and reduce the leak current by changing a channel length or a channel width of the TFT. However, if the circuit becomes finer, the change-by a scale as conventionally employed is limited.
For example, in order to obtain a high current driving ability it is necessary to increase the channel width. The monolithic circuit uses the TFT having a channel width of 500 to 1,000 xcexcm. However, if a higher current driving ability is required due to the increase in the number of pixels and the degree of gradation, it is difficult to further expand the channel width to 5 mm, 10 mm or the like from that the formation region of the peripheral circuit is limited.
On the other hand, it is desirable for the TFT used to control the pixel to obtain a clear image quality by increasing a charge retention ability. However, considering that the pixel region has a size of several hundreds xcexcm square, it is impossible to increase the channel length to 50 xcexcm, 100 xcexcm or the like in order to decrease the leak current. As result, since a scale of a matrix, a pitch and the number of pixels are largely limited in the conventional TFT monolithic type active matrix circuit, a displaying unit having a finer screen capable of obtaining a high quality image cannot be manufactured.
The above problems occur in not only the monolithic type active matrix circuit but also in other semiconductor circuits.
An object of the present invention is to overcome the problems and further improve the characteristics of a circuit as a whole.
The present inventor has confirmed that some metal elements are effective to promote crystallization of an amorphous silicon film. The elements which promote the crystallization are Group VIII elements such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt, 3d elements such as Sc, Ti, V, Cr, Mn, Cu and Zn, noble metal such as Au and Ag, and the like. Among the above, Ni, Cu, Pd and Pt have a large crystallization promoting effect. By adding those metal elements to the amorphous silicon film, the crystallization temperature can be lowered, whereby a time required for the crystallization can be shortened.
A method for adding the metal elements includes a method for forming the above-described metal element film or a thin film containing the metal element in contact with the upper or lower side of the amorphous silicon film. Further, it is confirmed that if the metal element is introduced by an ion implantation, substantially same effect is obtained. For example, it is confirmed that it is possible to lower the crystallization temperature in addition of nickel in an amount of 1xc3x971015 atoms/cm3 or more.
The amount of the metal element added varies depending on the type of the metal element. If nickel is used, it is desired that the amount thereof is in the concentration range of from 1xc3x971017 to 1xc3x971020 atoms/cm3. If the concentration of nickel is more than 5xc3x971020 atoms/cm3, nickel silicide is formed locally, resulting in deterioration of characteristics as the semiconductor. Further, if the concentration of nickel is less than 1xc3x971017 atoms/cm3, the effect of nickel as a catalyst is decreased. A reliability as the semiconductor becomes high as the nickel concentration decreases.
Thus, it becomes apparent that the crystallization can be promoted by adding specific metal elements to the silicon film. In addition, it is confirmed that by selectively adding those metal elements to the silicon film, a crystal growth selectively generates from a region to which the metal element has been added, and the crystal growth region expands into its periphery. Further, according to more detailed observations, needle crystals are growing in the direction along the substrate surface not in the direction in the thickness of the substrate, in the silicon film to which those metal elements have been added.
A crystal grows in a needle form in the silicon film to which those metal elements have been added. The width (length) thereof is about 0.5 to 3 times the thickness of the silicon film, and a growth in a transverse direction (a side direction of the crystal) is small. For this reason, a grain boundary is formed in parallel to the crystal growth direction. Where nickel is used as the metal element, the crystal grows in the (111) direction. An example of this crystal growth is shown in FIGS. 1A to 1C.
FIG. 1A is a top view, showing a state that a crystal growth generates from the region to which a metal element has selectively been added. Region 2 is a silicon film region to which the metal element has been added and the crystal growth expands from the region 2 to the periphery. Ellipse region 3 is region crystal-grown in a transverse direction. Arrows show the direction of the crystal growth. An outer region 1 outside the region 2 is a region which is not crystallized.
FIG. 1B is an enlarged view of a part of the region 3, for example, a square region 4. As is apparent from FIG. 1B, grain boundaries 6 and 7 generate in parallel to the direction of crystal growth (B to C) in the silicon film 5. Therefore, the grain boundary is less in a cross section (face BC) which is in parallel to the direction of the crystal growth, but many grain boundaries are observed in a cross section (face BA) which is vertical to the direction of the crystal growth.
In a case wherein such a film is oxidized by a thermal oxidation method, the thermal oxidation which can be employed includes a method of conducting a general thermal anneal in an oxidizing atmosphere (atmosphere of oxygen, ozone, nitrogen oxide, or the like), and a method of treating the surface of the silicon film at high temperature for a short period of time in an oxidizing atmosphere, as represented by a rapid thermal anneal (RTA) method.
The thermal oxidization proceeds along the amorphous silicon component-rich grain boundary. Therefore, as shown in FIG. 1C, an interface 9 between a silicon oxide layer 8 and the silicon film is markedly waved (uneven) in the face BA vertical to the direction of the crystal growth. However, the interface 9 is very smooth in the face BC which is in parallel to the direction of the crystal growth.
The above difference greatly affects an electric current flown on the surface of the silicon film. That is, a current flow is prevented by the unevenness of the interface 9 in the BA direction. On the other hand, a current flow is very smooth in the BC direction. For this reason, assuming that a direction to which a source/drain current flows in an insulating gate type field effect transistor which controls a current flowing a surface is the BA direction, the current flows as shown in a line 11 and the leak current is decreased by a substantial increase in the channel length. On the other hand, assuming that the source/drain current flowing direction is the BC direction, since there is no substantial barrier (grain boundary or the like), the current flows as shown in a line 10 and a mobility of this transistor becomes large. In particular, in order to sufficiently reduce the leak current in the BA direction as compared with the leak current in BC direction, it is desirable that the thickness of the thermal oxide film is 50 xc3x85 or more.
In particular, where an amorphous component is present in a crystalline silicon film, since the rate of oxidation is large in the amorphous component, an oxide film formed in a portion that the amorphous component is present (mainly, the vicinity of grain boundary) is thicker than that in the other portion. Therefore, where the unevenness of the silicon oxide film is considerably larger than the thickness of the gate insulating film, typically where the unevenness is 10% or more the thickness of the gate insulating film, an anisotropy on easiness of the current flow becomes remarkable.
By oxidizing the surface of the crystalline silicon film having the above anisotropy, and appropriately controlling the direction of the source/drain current of the silicon film, transistors having markedly different characteristics can be formed on the same substrate, and can also be formed adjacently. In actual transistors only the thermal oxide film is sometimes insufficient as the gate insulating film. In this case, an insulating film is further formed on the thermal oxide film by employing the conventional physical vapor deposition method (PVD method) or chemical vapor deposition method (CVD method).
As described above, the present invention is characterized in that (1) a metal element which promotes crystal growth of an amorphous silicon is selectively added to an amorphous silicon film, (2) a crystal growth having a directionality is conducted, (3) the crystallized silicon film is thermally oxidized, and (4) a TFT active layer is arranged such that an angle formed between a source/drain current direction and a crystallization direction has a predetermined angle xcex1. Furthermore, a plurality of TFTs each having different angle xcex1 are manufactured on the same substrate. Typically, various circuits can be constituted by using two kinds of TFT in the case of xcex1=about 0 (the crystal growth direction approximately coincides with the source/drain current direction (carrier moving direction) or the crystal growth direction is approximately parallel to the source/drain current direction) and in the case of a xcex1=about 90xc2x0 (the crystal growth direction is approximately vertical to the source/drain current direction).
For example, in the active matrix type liquid crystal display, the required characteristics differ between the TFT of the peripheral circuit and the TFT of the pixel portion. That is, it is necessary in the TFT which form a driver of the peripheral circuit to have a high mobility and flow a large on-current. On the other hand, in the TFT provided on the pixel portion, the mobility may not be high in order to increase a charge retentivity, but it is required that the leak current (off-current) is small.
The present invention uses a crystalline silicon film crystal-grown in the direction parallel to the substrate. In the TFT used in the peripheral circuit, the source/drain region is constituted in the direction parallel to the crystal growth direction. In the TFT used in the pixel, the source/drain region is constituted in the direction vertical to the crystal growth direction. That is, the TFT used in the peripheral circuit is constituted so as not to be influenced to the utmost by the grain boundary and the unevenness in the silicon film/silicon oxide film interface when a carrier moves. Moreover, the TFT used in the pixel is constituted so as to transverse the grain boundary when the carrier moves. By this constitution, a resistance between the source and the drain is high, and as a result, the leak current (off-current) is decreased.
Thermal oxidation is conducted to change the amorphous portion to a silicon oxide, the silicon oxide is etched with a buffer hydrofluoric acid and the like. This removes silicon oxide, thereby increasing a degree of the unevenness on the silicon surface. Subsequently, by further thermal oxidization, the unevenness in the silicon film/silicon oxide film interface can be further increased. Because an oxidation speed of the amorphous silicon is about 2 to 3 times that of the crystalline silicon, and the degree of the unevenness is further increased. As a result, difference on easiness of a current flow is further increased by an angle to the crystal growth direction.
The present invention can obtain the TFT having necessary characteristics by utilizing that the carrier flows between source/drain, and by making the source/drain direction (direction of line connecting the source and the drain) parallel or vertical to the crystal growth direction. That is, a TFT having a high mobility or a TFT having a small off-current is obtained by moving the carrier to the direction parallel to the grain boundary of the crystals grown in a needle form or a columnar form (direction parallel to the crystal growth direction), or the direction vertical to the grain boundary of the crystals grown in a needle form or a columnar form (direction vertical to the crystal growth direction).
Where the TFT is constituted using the crystalline silicon film crystal-grown in a direction parallel to the substrate surface, the TFT which has a high mobility and not so much influence of the grain boundary and the unevenness in the silicon film/silicon oxide film interface can be obtained by forming the source/drain region along the crystal growth direction. Further, the TFT which is affected by the grain boundary and the unevenness in the silicon film/silicon oxide film interface and therefore has a small off-current can be obtained by forming the source/drain region in the direction vertical to the crystal growth direction. The above TFT can be manufactured by optionally determining the direction of the carrier which moves between the source/drain, relative to the crystal growth direction.
FIG. 2 shows the embodiment manufacturing two kinds of TFT on a crystalline silicon region 14. The region 14 is a part of the ellipse crystalline silicon region 13 obtained by enlarging a rectangular region 12 to the periphery thereof. The crystal growth direction is indicated by an arrow 14a. The TFTs formed on the region 14 are TFT1 (source/drain regions 1a and 1c, and a channel formation region 1b) in which the source/drain direction is vertical to the crystal growth direction, and TFT2 (source/drain regions 2a and 2c, and a channel formation region 2b) in which the source/drain direction is parallel to the crystal growth direction. Typical characteristics of TFT1 and TFT2 are shown in FIG. 3. The on-current or off-current of TFT1 is small as compared with TFT2. For example, the off-current of TFT1 is typically 0.5 to 2 orders smaller than TFT2. Further, the on-current and the mobility of TFT2 is typically 10 to 30% large as compared with TFT1.
Therefore, if TFT1 is used in a pixel transistor of the monolithic type active matrix circuit and TFT2 is used in a driver transistor of the peripheral circuit, the characteristics of the active matrix circuit as a whole can further be improved.